Third Generation (3G) Networks such as UMTS (Universal Telecommunication Network) and CDMA 2000 provide high-speed wireless Internet access to mobile users over a wide coverage area. For the 3G networks the IP Multimedia Subsystem IMS has been defined to provide cellular access to the services of the Internet in order to support telephony and multimedia services. The IMS uses packet-based technology, in particular IP-network and other IETF protocols for provision of services. The strength of IMS is the provision of enhanced Services, for example multimedia services combining voice and data. Further, the usage of IP-network as a single underlying standard allows an easy and fast service deployment. In contrary, 2nd Generation networks, like GSM, provide voice based on a circuit-switched technology
A Session Initiation Protocol SIP has been chosen in IMS for signalling between the user's equipment UE and the IMS as well as between the components within the IMS. The IMS uses SIP also to complete voice and multimedia calls in the Internet. In order to be able to use the IMS service, the communicating user's equipment has to support IMS, which means SIP has to be implemented in the user's equipment.
In the following simplified network architectures of IMS is described. In particular the nodes being involved in provision of service in IMS architecture are mentioned.
The components of the IMS system are the Call Session Control Function (CSCF), the Media Gateway (MGW)/Media Gateway Control Function (MGCF), the Home Subscriber Register (HSR), the Application Server (AS).
The CSCF acts as a call server and handles call signalling, it supports and controls the multimedia sessions and performs address translation functions. The CSCF can be functionally decomposed to S-CSCF, I-CSCF and P-CSCF. The Proxy-CSCF (P-CSCF) is the first contact point in a visited IMS network and it provides authorization of bearer resources, further it forwards a SIP register request received from the User Equipment UE to an I-CSCF determined using the home domain name, as provided by the UE. In the opposite direction it forwards the SIP request or response to the UE. Moreover the CSCF forwards SIP messages received from the UE to a SIP server (S-CSCF) whose name the P-CSCF has received as a result of the registration procedure.
The Interrogating-CSCF (I-CSCF) is the contact point within an operator's network for all connections destined to a subscriber of that network operator, or a roaming user currently located within that network operator's service area. There may be multiple I-CSCFs within an operator's network. The main function performed by the I-CSCF is assigning a S-CSCF to a user performing SIP registration. The Serving Call Session Control Function (S-CSCF) is the node that performs the session management for the IMS network. There can be several S-CSCFs in the network. The main functions of S-CSCF include: acceptation of registration support of services. Further it provides endpoints with service event related information (like for example notification of tones/announcement together with location of additional media resources, billing notification).
The Home Subscriber Register HSR is the centralized subscriber database. The HSR interfaces with the I-CSCF and the S-CSCF to provide information about the location of the subscriber and the subscriber's subscription information. The HSR is responsible for holding the following user related information: user identification, numbering and addressing information, user security information for authentication and authorization. The HSR supports the user registration, and stores inter-system location information.
The IMS supports inter-working with legacy networks. This support is provided by the Media Gateway Control Function (MGCF).
The MGCF performs protocol conversion between cellular call control protocols and IMS protocols. For example, the MGCF receives a SIP message from the CSCF and converts it into appropriate ISUP messages. Thus, the primary function of MGCF is to convert signalling information from one format to another in uplink and downlink direction. In UMTS this will predominantly be between Pulse Code Modulation (PCM) in the PSTN and an IP based format.
As already mentioned the UMTS system allows mobiles operating in packet mode to establish voice calls using SIP as the signalling protocol. The SIP messages are sent to communicate the request to the Call Session Control Function (CSCF) in the IMS. In this case, the data is transmitted as packets throughout the UMTS network. However in order to access any service in IMS the user has to perform a registration procedure in the IMS system. Said registration procedure is performed by means of an user agent being implemented in the user's equipment.
Thus, the IMS has been deployed for the 3G networks for provision of services using packet-based technology with SIP as applied signalling protocol. However, currently the major numbers of user's equipment do not support IMS technology with SIP as signalling protocol for voice service, since said user's equipment is adapted for a circuit-switched controlled domain. Thus, for the access to the IMS an adaptation of the user's equipment is necessary.